In a wireless communication system, as a core element, a power amplifier has a function of amplifying a signal, so that the signal can be amplified to be at a sufficient power level, so as to implement transmission, long distance delivery and reliable receiving of the signal.
With the development of wireless communications technologies in modern times, in order to implement data transmission at a high code rate, some signals with a large dynamic range and a high Peak-to-Average Ratio begin to be used widely, for example, a Wideband Code Division Multiple Access (WCDMA) signal, and a typical value of a Peak-to-Average Ratio of the signal ranges 7 to 10 dB. Therefore, the power amplifier is required to have better linearity when peak power is output, and at the same time, is required to have high efficiency when average power is output.
A Doherty power amplifier is a widely used power amplification technology with high efficiency and low complexity currently. FIG. 1 is a structural diagram of a Doherty power amplifier in the prior art. The Doherty power amplifier generally includes two parts: a main power amplifier and a peak power amplifier. When input power is relatively small, the peak power amplifier is turned off. At this time, output impedance of the peak power amplifier is regarded as infinity, and output impedance of the main power amplifier is twice matched impedance. When a current reaches a half of a matched output current, the main power amplifier is saturated. At this time, power of the Doherty power amplifier reaches the peak, that is, saturation power of a class-AB power amplifier. As the input power is increased, the peak power amplifier is turned on. According to a load pull principle, the output impedance seen from a peak power amplifier port is increased. After passing through a ¼ wavelength microstrip, the output impedance of the main power amplifier is decreased, and output power of the main power amplifier may be increased continuously. Output voltage of the main power amplifier keeps the same, but the output power is increased continuously because a load is decreased. At this time, the main power amplifier still works in a saturation state, so that the Doherty power amplifier may still maintain higher efficiency.
The peak power amplifier is biased in a class-C state, and a gain of the peak power amplifier is lower, so that it cannot be ensured that the output power is the same as that of the main power amplifier when the saturation power is output. Since the output power is insufficient, a gain curve of the Doherty power amplifier has a step, thus influencing the linearity of the Doherty power amplifier during large power output. FIG. 2 is a curve diagram showing output of a Doherty power amplifier in the prior art.
In the prior art, in order to ensure that the peak power amplifier has sufficient power output, the peak power amplifier is generally turned on in advance before the main power amplifier is saturated. However, during implementation of the present invention, the inventors of the present invention find that the prior art has the following disadvantages. At this time, since the main power amplifier does not reach the efficiency peak, the turning the peak power amplifier on in advance causes that the efficiency of the whole Doherty power amplifier during power back-off is reduced. Furthermore, a certain static current is leaked out before the peak power amplifier is turned on, and the current generates power loss, thus influencing the efficiency of the Doherty power amplifier.